Method of increasing oil recovery



FIPTSIZZ XR 299949373 Aug. 1, 1961 H. L. STONE 2,994,373

7 METHOD OF INCREASING OIL RECOVERY Filed Dec. 18, 1957 2 Sheets-Sheet 1 men-r FLUID ssmuu'ron 27 l0 1 I a i 7 4; COMPRESSOR on.

l6 PRODUCTlON a v V A omelgm. OIL omen.- 20 2| ama? FLUID ZONE assznvom cm. w

0. 7 1 g i i 22% i i L 3 i f g ,sumcneu nou-enmcfin "-22 a 23 12 i 24 FLUID zous FLUIO zone I v V INVENTOR.

HERBERT L. STONE,

ATTORNEY.

c W K 1961 H. L. STONE 2,994g373 METHOD OF INCREASING on. RECOVERY Filed Dec. 18, 1957 2 Sheets-Sheet 2 3O 25 main FLUID SEPARATOR 29 26 coumssson 27 V A V Ll L2 ORIGINAL 12 4 nzsaavomon.

FLUID zou: FLUID ZONE '4 mam FLUID zone INVENTOR. FIG. 2. HERBERT L. STONE ATTORNEY.

United States Patent 2,994,373 METHOD OF INCREASING 03L RECOVERY Herbert Losson Stone, Houston, Tex, asslgnor to Jemey This invention is directed to a method for increasing.

recovery of oil from natural reservoirs. More particularly, the invention is directed to a method for increasing recovery of oil by simultaneous injection of two fluids, diflering in composition, into the reservoir.

Current oil production practices fail to recover much of the oil originally in place in natural reservoirs. As a consequence thereof, much effort has been devoted to devising methods of improving the ultimate recovery of the oil in the reservoirs. One such method for improving recovery of oil consists in displacing the reservoir oil with a hydrocarbon mixture as, for example, a hydrocarbon fluid usually containing methane and higher molecular weight hydrocarbon components. The solution of these components in the residual oil, especially the components less volatile than methane, leads to high oil recovery. An oil which has been diluted by .extensive solution of these components (that is, lighthydrocarbons such as ethane, propane, butane, pentane, etc.) or a natural gas which contains large quantities of these components is said to be enriched.

large volume of the hydrocarbon mixture, which volume is diflicult and expensive to'obtain.

Most natural reservoir oils have dissolved in them large quantities of light hydrocarbons, such as methane, ethane, propane, butane, pentane, etc. When these lighter hydrocarbons are separated from heavier hydrocarbon components in a separator or distillation column, the resulting more volatile lighter component mixture is of a composition suificiently enriched with intermediate hydrocarbons (ethane, propane, butane, etc.) as to give high oil recoveries. However, when oil is produced from a reservoir and separated into a light component phase and a heavy component phase and the light component phase compressed and reinjected into the formation, the reservoir pressuredeclines because of the removal of the heavy component phase. A pressure decline in the reservoir interferes with ultimate recovery 7 of the oil.

In someinstances it may be desirable to employ lighter hydrocarbon components obtained exterior of the system. Therefore, the method of the invention encompasses using light hydrocarbon components whether thecomponents are obtained from the reservoir oil or whether they are obtained, all or in part, from a source exterior of the system. ,1

Briefly, the invention comprises the steps of injecting light hydrocarbon components into a reservoir through a first injection well, the first injection well being spaced from the production well, injecting an inert fluid into the reservoir through a second injection well, the second injection well being spaced from the first injection well and the production well. a

For. purposes. herein the inert fluid is defined as a fluid ice Additional objects of this invention will be apparent from a. description of the invention taken in conjunction with the drawings wherein:

FIG. 1 is a schematic view illustrating one embodiment of the invention; and

FIG. 2 is a schematic view illustrating another embodiment of the invention.

Referring to FIGS. 1 and 2 more specifically, numeral 10 designates the surface of the earth upon which are arranged the uppermost portions of injection wells 11 and 12 and a production well 13; a separator 15; a pump 16; and a compressor 17. An oil reservoir 14 is shown located below the surface of the earth 10. Injection wells 11 and 12 and production well 13 extend below the surface of the earth 10 to reservoir 14. Production well 13 fluidly connects with separator 15, which, in turn, connects to pump 16 and compressor 17, as shown.

As illustrated in the drawings a light hydrocarbon mixture is being injected through injection well 11, an inert fluid is being injected through injection well 12 and oil is being produced through production well 13.

With this operation, in accordance with the embodiment shown in FIG. 1, an enriched fluid invaded zone 20 is created adjacent injection well 11; a non-enriched fluid invaded zone 21 is located adjacent zone 20; an original reservoir oil zone 22 is located between injection well 12 and zone 21; an oil and inert fluid invaded zone 23 is located adjacent injection well 12 and an original reservoir oil zone 24 is located adjacent production well 13. Zones 22 and 24 may or may not exist depending upon the length of time the operation has been conducted.

The embodiment illustrated in FIG. 2 is similar except production well 13 is positioned between injection wells 11 and 12. Thus, for the operation, described supra, enriched fluid invaded zone 20 is formed adjacent injection well 11 and non-enriched fluid invaded zone 21 is located adjacent zone 20. However, original reservoir oil zone 22 is positioned between production well 13 and zone 21 and an oil inert fluid invaded zone 23 extends' from injection well 12 to zone 22. V

In both embodiments reservoir oil may be produced from production well 13 and sent to separator 15 where the oil is separated into a fluid containing the lighter hydrocarbon components and a liquid containing the heavier hydrocarbon components. If the separated fluid is to be used as the enriched injection fluid, it is sent to a compressor 17 from separator 15, as indicated by arrowed line 25, wherein the fluid phase is compressed to a pressure suflicient to displace the reservoir oil. From compressor 17 the enriched fluid is sent to injection well 11, as indicated by arrowed line 26. The liquid is removed from the system, as indicated by arrowed line 27. Inert fluid is sent to a compressor or pump 16, as indicated by line 29 where the inert fluid is compressed or-pumped to a pressure suflicient to displace the reservoir oil. From compressor or pump 16 the inert fluid is sent to injection well 12, as indicated by arrowed line 28.

In the event the enriched fluid mixture injected through injection well 11 is obtained from a source external of the system, the method is the same except that all or a portion of the enriched fluid injected is sent to injection well 11 from a source of enriched fluid, not shown, as indicated by arrowed line 30.

For the FIG. 1 embodiment the optimum location of injection well 12 is obtained in the following manner:

The practical size of the enriched fluid bank (zone 20) is limited by two factors (1). After the inert fluid-original oil mixture (zone 23) reaches production well 13, the produced fluid will yield a gas phase diluted by inert fluid. Injection connot be continued except with enriched fluid 3 from an outside source and even then it will involve injetcion of large quantities of inert fluid since much of this fluid is now being produced. Therefore, to determine the optimum location of injection well 12, it is assumed that the injection of inert fluid into the reservoir will be stopped when inert fluid begins to be produced by production well 1-3. (2) When the enriching components of zone 20 reach injection well 12, any further growth of this zone will be prevented by dilution of the enriched fluid by the istics. A fluid hydrocarbon mixture of the same composition to be injected through injection well 11 is injected into one end of the model reservoir and oil is displaced and produced at the other end until the injected enriched fluid is first produced at the other end of the reservoir. The ratio (E) of the total volume of fluid injected per unit cross-sectional area of the experimental reservoir to the length of the experimental reservoiris evaluated at the time the injected enriched fluid is first produced.

Similar experiments are conducted wherein an inert fluid of the same composition as the inert fluid to be injected through injection well 12 is substituted for the fluid hydrocarbon mixture of the same composition as the fluid hydrocarbon mixture to be injected through injection well 11. The ratio (K) of the total volume of inert fluid injected per unit cross-sectional area of the experimental reservoir to the length of the experimental reservoir is evaluated at the time of inert fluid breakthrough.

For purposes of calculation a linear system is considered where L; is the distance between injection well 11 and injection well 12; I; is the length of zone at any time; 1., is the distance between injection well 12 and production well 13; I, is the length of zone 23 at any time; L is the total length of the reservoir 4: is the porosity of the reservoir; G is the cumulative volume (at reservoir conditions) of fluid injeotul into well 11 per unit of cross-sectional area; 0 is the volume of inert fluid (at reservoir conditions) injected through injection 12 per unit of crosssectional area of the reservoir; then at any g'ven time and When zone as reaches'well 12 and when none 23 reaches injection well 12 and Therefore,

.ia., L .2 .22 K GK Similar relationships to determine the optimum location of injection well 12 can be derived for non-linear reservoirs. The value L represents the minimum desirable value in all cases. In some instances it maybe desirable to rel-ax the first restriction if enriched fluid can be obtained from an exterior source, as noted supra. Relaxing this restriction will permit L to be made somewhat larger than indicated by Equation 1, but this increase will be limited by the fact that large quantities of inert fluid will have to be injected into well 12 to make up for the large inert fluid production from well 13.

In the above embodiment the calculations for determining the position of the inert fluid injection ,well is the same whether or not any or all of the enriched fluid injected through injection well 11 is obtained from a source exterior of the system.

In the embodiment shown in FIG. 2, the relative locations of production well 13 and injection well 12 are reversed. Thus production is from the center of the reservoir and an inert fluid is injected into the end of the reservoir opposite the end wherein the enriched fluid is injected. The optimum location of -the production well in the center of the formation is determined as follows:

In the instance where the enriched fluid injected through injection well 11 is obtained solely from the produced oil, it is not desired to dilute the produced oil with inert fluid or the non-enriched gas in zone 21. Therefore,

for obtaining the optimum location of injection well 12,, it is assumed that the inert fluid injected through injection well 12 reaches production well 13 at the same time that the non-enriched gas in zone 21 reaches production well 13. An additional factor (F) is employed which is similar to (E); that is, it is the ratio of the total volume of fluid injected per unit cross-sectional area of the experimental reservoir to the length of the experimental reservoir. However (F) is evaluated at the timetof stripped gas (gas in equilibrium with the reservoir oil) breakthrough rather than at the time injected enriched fluid is first produced. Also, the length of zone 21 is designated 1;. The location of the central production we 13 and the length of the enriched zone I, are determined as follows:

I =GIE Since zones 21 and 23 reach well 13 simultaneously, then E OF a; and the length of enriched zone 20 is and L E0 1+ GK Further, the restriction that the inert fluid injection be halted when the inert fluid begins to be produced in production well 13 may be relaxed. This permits moving the inert fluid injection well 12 downstream and hence will permit build up of a larger zone of enriched gas. This is limited by the largequantities of inert fluid which must be injected since when inert fluid begins to be produced the inert fluid injection rate must be increased an amount equal to the rate of inert fluid production.

At any desired time the operation may be halted and extraneous gas injection initiated at injection well 11. This gas or fluid may be rich in intermediates or it may be a lean gas consisting principally of methane or even of more insoluble materials such as air or nitrogen.

and

Whatever type gas is injected, recovery of oil will be en-.

hanced by the presence of the enriched zone 20.

For purposes of illustration the description herein refers to single fluid injection, liquid injection and productionweils. However, it is to be understood the invention encompasses the more practical arrangement wherein a plurality of fluid injection, liquid injection and production wells are used.

Having fully described the nature, objects and operation of my invention, I claim:

1. A method for increasing recovery of oil from a subsurface reservoir comprising the steps of producing oil from said reservoir through at least one production well while simultaneously injecting an enriched hydrocarbon mixture containing large quantities of low molecular weight hydrocarhons'into said reservoir through at least one first injection well in order to displace reservoir oil, and an inert fluid into said reservoir through at least one second injection well in order to inhibit reservoir pressure decline, said second injection well being located between said first injection well and said production well and said wells being selectively spaced from each other such that said enriched hydrocarbon mixture injected through said first injection well reaches said second injection well at approximately the same time said inert fluid injected through said second injection well reaches said production we.

2. A'method for increasing recovery of oil from a subsurface reservoir comprising the steps of producing oil from said reservoir through at least one production well while simultaneously injecting an enriched hydrocarbon mixture containing large quantities of low molecular weight hydrocarbons into said reservoir through at least one first injection well in order to displace reservoir oil, and an inert fluid into said reservoir through at least one second injection well in order to inhibit reservoir pressure decline, said production well being located between said first and second injection wells and said wells' being selectively spaced from each other such that said enriched hydrocarbon mixture injected through said first injection well reaches said production well at approximately the same time said inert fluid injected through said second injection well reaches said production well.

References Cited in the file of this patent Great Britain Mar. 23, 1955 

